US3692645A - Electrolytic process - Google Patents

Abstract

PROCESS FOR THE ELECTROLYSIS OF MOLTEN OXIDES, ESPECIALLY OF ALUMINA, IN WHICH A GAS PERMEABLE ANODE IS SEPARATED FROM THE MELT BEING ELECTROLYZED BY A LAYER IN CONTACT WITH THE ANODE AND THE MELT, OF AN OXYGEN-ION-CONDUCTING MATERIAL, FOR EXAMPLE ZIRCONIUM OXIDE STABILIZED WITH CALCIUM OXIDE OR OTHER OXIDES, WHICH IS RESISTANT TO THE MELT AT THE TEMPERATURE OF THE ELECTROLYSIS.

Description

United States Patent US. Cl. 204-67 1 Claim ABSTRACT OF THE DISCLOSURE Process for the electrolysis of molten oxides, especially of alumina, in which a gas permeable anode is separated from the melt being electrolyzed by a layer, in contact with the anode and the melt, of an oxygen-ion-conducting material, for example zirconium oxide stabilized with calcium oxide or other oxides, which is resistant to the melt at the temperature of the electrolysis.

CROSS REFERENCE TO RELATED APPLICATION This application is a division of my copending application Ser. No. 638,249, filed May 15, 1967, now U.S. Pat. 3,562,135, patented Feb. 9, 1971.

BACKGROUND OF THE INVENTION The electrolysis of molten materials, for example of alumina, is carried out today with carbon anodes. In the case of alumina, the oxygen ions formed during the electrolysis react with the carbon of the anode at the process temperatures of 900 to 1000 C. and form carbon dioxide, which is partly reduced to carbon monoxide by the aluminium itself. Owing to the oxidation of the anode by the nascent oxygen, the carbon anode is consumed and, in fact, if only carbon dioxide were to be formed, 334 kg. of carbon per ton of aluminium produced would be consumed. In practice, about 400 to 450 kg. of anode carbon are consumed per ton of aluminium, which corresponds to about 8 to 10% of the cost of crude aluminium. It has only recently been possible to reduce the consumption of anode carbon even to this figure, and with the present method of working using carbon anodes, reduction of the consumption of anode carbon below the theoreticall smallest amount, that is 334 kg. of carbon per ton of aluminium, is not possible.

It has now been found that it is possible to carry out the melt electrolysis of oxides without using carbon anodes, but using electrodes which are oxygen-resistant Without necessarily being resistant to attack by the melt being electrolyzed, so that the oxygen can be obtained as a valuable by-product of the process. About 600 mm. of pure gaseous oxygen are formed per ton of aluminium; the value of the oxygen is about 3% of the cost of the crude aluminium. The oxygen which can be recovered in carrying out the process according to the invention into effect can be used for various oxidising processes, such as for example, steel production (by the oxygen blow method), the gasification of fuels (for producing synthetic gas) and the preparation of reducing gases for iron reduction.

According to the invention I use an anode of any suitable conducting material, and I separate this anode from the melt being electrolysed by a layer of material which is oxygen-ions-conducting but non-permeable to and resistant to the melt at the temperature of the electrolysis, so that the oxygen ions diffuse through the layer and are then discharged at the anode with the formation of oxygen gas. The anode itself preferably consists of an electronice conducting material which does not react with oxygen or at least does not form with oxygen any compound impairing the conduction of electrons. Suitable materials include heat-resistant alloys, platinum or other noble metals, electron-conducting oxides, such as for example, wiistite, certain materials with semi-conductor properties, and metals with a passivated surface. The thickness of the oxygen-ion-conducting layer may be very small so that the voltage drop across it is also small; this reduces losses of energy during the electrolysis.

SUMMARY I have found that know stabilised forms of zirconium oxide are very suitable as the material which separates the anode from the melt. By stabilised I mean zirconium oxide in which is incorporated proportions of other oxides such as calcium oxide, magnesium oxide and yttrium oxide, which serve firstly to stabilise the cubic (fluorite) lattice of the zirconium oxide, and secondly to confer on it the necessary oxygen-ion conductivity. By suitable choice of oxides and their proportions, a stabilised zirconium oxide can have a resistance as low as 10 ohms-cm. at 1000 C. Other refractory oxides which have fluorite lattices can be used, such as for example, rare earth oxide-uranium oxide compositions, thorium oxide-uranium oxide compositions and cerium oxide suitably stabilised with calcium oxide or magnesium oxide. Substances which reduce the solubility of the oxygen-ion-conducting material may be added to the fused melt.

The invention will be described hereinafter with specific reference to the electrolysis of alumina for the production of aluminium. In such an electrolysis, the oxygen ions which are formed in accordance with the equation dilfuse through the oxygen-conductive layer and are discharged at the anode in accordance with the equation i.e. the oxygen ions combine to form oxygen gas and electrons are released in the process. These electrons are conducted away by the anode. Other oxides such as for in stance, MgO, Na O, CaO, Fe O can also be electrolysed by the process according to the invention and similar equations can be formulated. In the electrolysis of alumina for example, cells according to the invention atford the following advantages, inter alia, in comparison with the present state of the art. I

(1) There is no consumption, or only a very low rate of consumption, of anode material with the result that the rate of production of anode material can be substantially reduced.

-(2) The formation of carbon scum in the bath results in a loss of operating efficiency, and this formation will not occur if the anodes are of material other than carbon.

(3) There is improvement of the quality of the aluminium metal produced, since no impurities, such as iron, silicon or vanadium are introduced from the anodes material.

(4) There is less down-time of the cell, since the anodes do not have to be replaced.

(5) There is a reduction of the consumption of fiuxing materials, since the cell can be sealed off more satisfactorily and this also gives an improvement in the shop atmosphere.

(6) Pure oxygen can be produced and collected as a byproduct.

(7) There is no re-oxidation of the liquid aluminium by carbon dioxide and thus, there is increased output from the cell and a reduction in the energy required to produce a given weight of aluminium.

Cells according to the invention can readily be adapted for automation of operation with for example, continuous addition of alumina to the fused melt and maintenance of constant interelectrode gap or cell voltage.

Cells according to the invention may be constructed in two Ways. In the first of these, the anode is coated with or is in contact with the layer of oxygen-ion-conducting material over at least that part of its surface which is immersed in the melt; the anode must then be in such a physical state that oxygen gas can pass through it.

The anode may be solid, in which case it must be porous, perforated or reticulated.

If the anode is solid, the layer of oxygen-ion-conductin'g material may be applied directly to it by pressing or casting with subsequent drying and sintering or by plasma spraying. Alternatively a body of the material may be preformed quite separately and put in contact with the anode, if the latter is, for example, a metal network. As a further possibility, a porous layer of platinum black may be applied to a preformed body of the material, and electrically connected to one terminal of the current supply. This last proposal is found to be very satisfactory, as platinum black is particularly suitable for the discharge of oxygen ions and the formation and removal of oxygen gas.

What is claimed is:

1. The method of reducing molten oxide by electrolysis in an electrolytic cell which comprises passing a direct electric current between a cathode in contact with the melt and a gas permeable anode resistant to the formation with oxygen of any compound impairing the conduction of electrons, said anode being separated from the molten oxide of the melt by a layer of solid material in contact with the anode and immersed in and in contact with the melt and which is oxygen-ion-conducting but non-permeable to and resistant to the material of the melt at the temperature of the electrolysis, and said current as it passes through the cell causing diffusion of oxygen-ions through said layer of material with the formation of oxygen gas References Cited UNITED STATES PATENTS 6/1971 Stewart 20464 X 9/1968 Tasiri et a1. 204--11 JOHN H. MACK, Primary Examiner N. A. KAPLAN, Assistant Examiner U.S. Cl. X.R. 20464 R,